SES V6 BRAKE PROBLEMS

2001 Ford Taurus

I replaced my rear drum brakes a while back and had to get new drums and new shoes. They make an awful screeching/metalic grinding sound that I can't explain when I press the brake pedal. It's been nearly a year and they don't seem to be worn much at all. The disc brakes in the front are working perfectly still and are surprisingly not that worn either.

I thought the shoes were loose, so I tightened them and it would help for 2 days and start again. Then I checked them again and tried loosening them which lasted about 2 weeks before starting again.

I've been checking these brakes nearly every weekend, and sometime in there I found one shoe that was cracked across the middle. They were from Oreily's and they replaced all 4 shoes after some convincing. He said that crack could ONLY happen if it was too tight, which I don't really believe.

Each time, I tried using sand paper to scuff the drums up to remove any glazing. After it helping some, I tried using sand paper on the shoes too and it seemed to help some. But the sould ALWAYS comes back! It's driving me nuts!

Can worn out wheel cylinders cause that? Or master cylinder. Or something else? The proportioning valve is pretty expensive and Autozone and Oreily's both say that those very rarely go out.